The erection of above-ground structures often requires the formation of in-ground retaining walls for use as a load-bearing foundation or as a barrier to prevent the collapse of soil into the excavated area. Where such excavations are made adjacent an existing structure, a retaining wall along the excavation line adjacent the existing structure is necessary to resist soil pressures established beneath the adjoining structure. If no retaining wall is formed, the soil beneath the adjoining structure can collapse outwardly into the excavation and/or damage the existing structure. In addition to retention of soil, retaining walls of this type are often constructed to block the flow of ground water into the excavated area.
A number of methods have been employed to form retaining walls around an excavation site or adjacent structures which adjoin such site. One method has been to employ piles formed of wood or steel which are driven along the excavation line to form the retaining wall. Alternatively, a row of bored holes are formed along the excavation line which receive reinforced concrete piles to form the retaining wall. Both of these methods produce retaining walls which are not water-tight, and which may require substantial horizontal strengthening to maintain the desired alignment along the excavation line.
Trenching apparatus such as disclosed in U.S. Pat. No. 4,843,742 to Camelleri have been proposed as an alternative to the methods and apparatus of forming retaining walls mentioned above. In trenching apparatus of this type, a supporting base capable of being moved along an excavation line carries an elongated trench digger arm supported on one side thereof by a boom assembly. The support base is drivingly connected to skids or track assemblies of the type employed in conventional bulldozers which are effective to move the support base and digger arm along the excavation line to form a trench of the desired depth. Concrete is poured into the trench immediately behind the moving trenching apparatus into which appropriate reinforcing bars are inserted so that an essentially continuously formed, reinforced concrete retaining wall is provided at the excavation site.
The trenching apparatus described in U.S. Pat. No. 4,843,742 includes a boom assembly having an inner boom which slides in and out of a outer boom by operation of linear actuators such as hydraulic or pneumatic extension cylinders. The housings of the linear actuators are carried by the outer boom, and their actuating rods are connected to the inner boom. The digger arm is pivotally connected to and suspended from the end of the inner boom, and the bottom of the outer boom is pivotally connected to the support base.
One problem with trenching apparatus of the type described in U.S. Pat. No. 4,843,742, particularly in digging deep trenches and/or in hard and rocky soils, involves damage to the linear actuators resulting from twisting, bending and vibration of the boom assembly during operation and manipulation of the digger arm. Not only does the boom assembly support the heavy weight of the digger arm at the end of the inner boom, but additional forces are exerted against this structure while digging a trench. Because the linear actuators in these apparatus are rigidly connected to the booms, they also twist, bend and vibrate along with the booms during the trenching operation. As a result of the stresses generated by this movement, the linear actuators can become damaged.